The present invention relates to a dynamic-pressure machine for charging internal-combustion engines.
The measures by means of which attempts are made to improve the efficiency of dynamic-pressure machines include reducing the plays between the end faces of the rotor body and the gas housing or the air housing. To keep the leakage losses as low as possible, attempts are made to keep these plays as small as possible, and these should remain as constant as possible over the entire operating range.
Attempts are made to ensure that these conditions are adhered to, by selecting suitable materials which are co-ordinated with one another in respect of their coefficients of thermal expansion, but which, at the same time, have to withstand the thermal and dynamic stresses occurring during operation. This is especially true of the rotor for which only high-temperature materials are suitable.
Till recently, among other things an Invar alloy of high heat resistance and, up to a temperature of approximately 350.degree. C. (the Curie point), of uniformly low coefficient of thermal expansion has proved suitable for this purpose. However, above this temperature the coefficient of thermal expansion increases abruptly, so that the charging efficiency decreases sharply unless special constructive measures making production more expensive are taken. Consequently, this alloy is suitable only to a limited extent for these higher temperatures. In the striving for even higher exhaust-gas temperatures, for example in gasoline engines, even special alloy steels or metallic superalloys no longer meet the requirements mentioned.
The demand for as small plays as possible between the rotating and the stationary components over the entire load range of the engine can be met only very inadequately with the materials used hitherto for this purpose. For in the case of rapid load changes, the rotor always undergoes the quickest temperature change and, consequently, change in diameter and length. The other parts experience the change in temperature and, consequently, in their dimensions with a delay, so that the plays can temporarily be cancelled completely--during acceleration, the rotor thus beginning to scrape, or--during throttling--the rotor cools more quickly, the plays consequently become temporarily very large and the efficiency decreases accordingly.
To keep these changes in play within as narrow limits as possible, the wall thickness of the rotor housing is made as thin as possible, so that the latter is heated and cooled rapidly during load changes and can consequently follow sufficiently quickly the rapid changes in length and diameter of the rotor. However, small wall thicknesses of the rotor housing signify greater heat losses and therefore a loss of efficiency.
The present invention arose from the object of finding a design for the rotor and the rotor housing of a dynamic-pressure machine, in which the disadvantages described above are avoided. In other words, under all operating states and especially during load changes, uniformly low values are maintained for the plays between the rotor end faces and the end faces of the gas or air housing, in order to prevent scavenging losses or scraping, and in which, because of a higher thermal loading capacity, a greater efficiency can be obtained and better acceleration ability is achieved.
This is put into practice, according to the invention, by the use of ceramic materials for the rotor and the housing and by a constructive shaping, adapted to the properties of this material which is novel for dynamic-pressure machines, of these parts and of the means for connecting them to the rotor shaft or to the gas and the air housing.